We seek to understand replication and partition mechanisms that account for the stable inheritance of unit-copy bacterial plasmids. The P1 plasmid prophage chosen for study is representative of a class of genetic elements characterized by the presence of iterated DNA sequences at the origin of plasmid replication and in a replication control region. It is also characterized by a specialized partition apparatus. In the period of this report our laboratory has provided conclusive evidence for the existence of a novel biological control mechanism in which the control region acts in trans without mediation by messenger RNA. Means of testing the validity of alternative schemes by which this control mechanism works have been devised. We are also interested in the roles played by alternative origins in P1. From P1 phages that cannot replicate from their normal dnaA-dependent origin, insertion mutants which fail to replicate from an alternative dnaA-independent replicon have been obtained. Surprisingly the phage is still proficient for replication, indicating the presence of yet another origin. Preparatory to the isolation of bacteria with altered capacity for plasmid partitioning, Lambda-miniP1 hybrids have been constructed bearing markers that permit selection, counterselection and visual discrimination of mutants.